Wireless systems divide a large service area into a number of smaller discrete geographic coverage areas called “cells.” Each cell is contiguous with adjacent cells to provide continuous coverage throughout a service area. A base station having a plurality of transceivers is provided for each of the cells within the service area. FIG. 1 illustrates elements residing in a typical cell, including a base station 10 and mobile stations 12.
In a time division multiple access (TDMA) system, a mobile station 12 transmits a burst of an information-bearing signal, such as a coded speech signal, at a carrier frequency within a designated time slot, the time slot occurring within a defined frame of fixed duration. A single time slot reoccurs at specified time intervals in subsequent frames. A communication channel is defined in terms of a particular time slot and a carrier frequency. While one mobile station 12 in a system is transmitting in one time slot, other mobile stations 12 may operate at the same frequency using the other time slots within the frame.
Radio frequency (RF) interference in a wireless system degrades the quality of the communications between the base station 10 and the mobile stations 12 within the cell. In order to compensate for higher interference levels within a cell, the base station 10 and the mobile stations 12 may transmit at a higher power level, so that interference does not overwhelm the signals transmitted by the base station 10 and the mobile stations 12. However, while a higher transmit power level may compensate for high interference levels, the use of higher transmit power levels adds further RF interference to the environment, and places increased power demands on the base station 10 and the mobile stations 12. Cell site engineers must therefore balance the need for a minimum voice quality with the adverse effects associated with increased transmit power levels.
A measure of quality for voice communications may be expressed as the ratio of signal carrier power level (or, carrier level C) to the interference level I in the cell, known as the carrier-to-interference (C/I) ratio. As cell capacities increase, and voice communication quality decreases, cell site engineers can compensate for interference by adjusting cell coverage based upon C/I ratios. A C/I ratio can be used as a handoff trigger, or as a flag to cell site operators that the RF coverage plan may require reengineering.
In order to properly adjust cell coverage, cell site engineers must have access to accurate values for both the carrier power level and the interference level I. Carrier power levels are relatively easy to obtain because the base station 10 continuously measures the carrier power level of signals received from the mobile stations 12. It is more difficult, however, to obtain accurate interference level I values.
A conventional method of measuring interference includes taking static interference measurements at certain times of the day during an inactive TDMA time slot. A channel, and its reoccurring time slot, are “active” when a mobile station 12 assigned to the channel is in communication with a base station 10 during the channel's time slot, the continued communication over successive frames being known as a “call.” The conventional interference measurements on inactive time slots are “static” measurements because they are made at predetermined times of the day. Because data is not transmitted in the inactive time slot, a measure of received power during the time slot reflects the level of interference during the time slot. A problem with static interference measurements is that interference dynamically changes during a call, and static interference measurements do not reflect these dynamic changes.
FIG. 2 illustrates another conventional technique for measuring interference in a TDMA system. In FIG. 2, time slots 1 and 3 are transmitting an active call (carrier ON), while time slot 2 is inactive (carrier OFF). An interference measurement may therefore be taken during the inactive time slot 2. While this technique provides an accurate measure of the interference level I during the inactive time slot 2, and may be used at any time of the day, it may only be used when a time slot is inactive. This is a serious shortcoming because interference levels are most likely to vary during the busiest periods of the day, when the fewest number of times slots are available for interference measurement.